1. Field of the Invention
The present invention relates to a method of efficiently operating a direct current (DC) generator. More particularly, this invention relates to a method of using power curve characteristics of a DC generator to operate the generator efficiently.
2. Description of the Related Art
A DC generator (such as a photovoltaic (PV) cell, a fuel cell, a wind turbine, or a microturbine, for example) has a polarization curve that represents a relationship between voltage and current generated by the generator. The polarization curve varies depending on the operating conditions of the DC generator.
FIG. 1A is a generalized polarization curve for a PV cell. As shown in FIG. 1A, the polarization curve of a PV cell varies depending primarily on cell temperature and on an amount of solar radiation incident on the cell. In a DC generator, PV cells can be interconnected together to form a stack or array having a higher power capacity. The stack or array, however, retains the same characteristic polarization curve.
A polarization curve can be converted into a power curve using the relationship:Power=Voltage×CurrentFIG. 2A is a graph illustrating a power curve for a DC generator. Referring to FIG. 2A, a power curve is a graph representing the relationship between power and either voltage or current with respect to a given set of operating parameters. The power curve (expressed in terms of either voltage or current) has a global maximum, referred to as a maximum power point (MPP). Although the specific voltage or current at which the global maximum occurs changes as the shapes of the polarization and power curves change with operating conditions, the point is always defined the same way. On the power curve, for example, the MPP is typically the point at which the slope of the curve equals zero (0). On the polarization curve, the MPP is generally the point at which the percentage change in current and voltage are equal but opposite.
To extract the maximum power possible from the DC generator, the operating current and voltage of the generator should be controlled in such a way as to operate as close as possible to the global maximum at all times. This principle, called Maximum Power Point Tracking (MPPT), has been applied successfully in PV systems.
Conventionally, MPPT is performed using a perturb and observe method. In this method, the voltage and current of a photovoltaic cell are measured while an operating voltage is varied. A power output is calculated using the measured voltage and current. The voltage is, for example, first decreased until the measured power begins to decrease. The voltage is then increased until the measured power begins to decrease again. These steps are continuously repeated. In this manner, the operating point of the photovoltaic cell is constantly varying, but always remains very near the global maximum of the power curve. This method is also able to track changes to the global maximum that occur as a result of variations in operating conditions.
A variation on this technique includes observing and analyzing DC voltage and current ripple. Another variation includes occasionally disconnecting the generator from the electrical power system temporarily while using a separate circuit to trace the full polarization curve.